1. Field of the Invention
The present invention relates to a fuel cell having a power generation unit including a first metal separator, a first electrolyte electrode assembly stacked on the first metal separator, a second metal separator stacked on the first electrolyte electrode assembly, a second electrolyte electrode assembly stacked on the second metal separator, and a third metal separator stacked on the second electrolyte electrode assembly.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane, and interposed between an anode and a cathode to form a membrane electrode assembly (electrolyte electrode assembly). Each of the anode and the cathode includes an electrode catalyst layer and porous carbon. The membrane electrode assembly is sandwiched between a pair of separators (bipolar plates). The membrane electrode assembly and the separators make up a unit cell for generating electricity. In use, typically, a predetermined number of unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field (reactant gas flow field) for supplying a fuel gas is formed on a separator surface facing the anode, and an oxygen-containing gas flow field (reactant gas flow field) for supplying an oxygen-containing gas is formed on a separator surface facing the cathode. Further, as necessary, a coolant flow field for supplying a coolant along separator surfaces is formed between separators.
In some designs, the coolant flow field is provided for every predetermined number of unit cells for so-called skip cooling to reduce the number of the coolant flow fields, and reduce the overall size of the fuel cell stack in the stacking direction.
For example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2000-223137 shown in FIG. 11 includes a separator 1A, an electrode unit 2A stacked on the separator 1A, a separator 1B stacked on the electrode unit 2A, an electrode unit 2B stacked on the first separator 1B, and a separator 1C stacked on the electrode unit 2B. Each of the electrode units 2A, 2B includes a fuel electrode 2b, an air electrode 2c, and a solid electrolyte membrane 2a interposed between the fuel electrode 2b and the air electrode 2c. 
Each of the separators 1A to 1C has a fuel gas flow field 3a on a surface facing the fuel electrode 2b, and has an oxygen-containing gas flow field 3b on a surface facing the air electrode 2c. 
Each of the separators 1A to 1C has ridges 4 protruding in one direction formed by corrugating a metal plate. The ridges 4 contact the fuel electrode 2b to form the fuel gas flow field 3a. Further, each of the separators 1A to 1C has ridges 5 protruding in the other direction. The ridges 5 contact the air electrodes 2c to form the oxygen-containing gas flow field 3b. 
However, in the fuel cell, when the electrode unit 2A is sandwiched between the separator 1A and the separator 1B, the ridges 4 and the ridges 5 are offset from each other in the stacking direction indicated by an arrow S. In the structure, the electrode unit 2A cannot be sandwiched securely between the separators 1A and 1B. Thus, it is difficult to apply the desired tightening load to the electrode unit 2A. Consequently, power generation cannot be performed efficiently, and the electrode units 2A, 2B may be damaged undesirably.